Methods of changing biopolymer volume fraction and cytoplasmic solute concentrations for in vivo biophysical studies

In vitro changes in polymer volume fraction (macromolecular crowding) and changes in solute or salt concentration typically have large effects on protein and nucleic acid processes (e.g., folding, binding, assembly, precipitation, crystallization). However, the large changes in these concentration variables, which occur in vivo as part of cellular responses to osmotic stress, appear to have much less dramatic effects on cellular biopolymer processes. Methods of changing intracellular concentrations by varying the extracellular osmolality or the concentration of a permeable solute or by titrating cells with an impermeable solute (plasmolysis) under conditions where an active response is suppressed are reviewed. The first in vivo biophysical studies of protein folding and protein diffusion performed as a function of these variables are also discussed.